improve testing of material parameter conversion functions
improve testing of material parameter conversion functions

- rename test_tensors() -> test_elastic_constants()
- new test_conversion_functions(), test_stiffness_tensors()

RexFuzzle/sfepy,sfepy/sfepy,lokik/sfepy,sfepy/sfepy,vlukes/sfepy,BubuLK/sfepy,vlukes/sfepy,lokik/sfepy,RexFuzzle/sfepy,rc/sfepy,vlukes/sfepy,RexFuzzle/sfepy,rc/sfepy,lokik/sfepy,rc/sfepy,sfepy/sfepy,BubuLK/sfepy,lokik/sfepy,BubuLK/sfepy,RexFuzzle/sfepy